1. Field
A device, such as a flat panel display (hereinbelow, called an FPD), is fabricated by using photolithographic technology to project with high accuracy a mask pattern formed on a photomask to a front surface of a device substrate, thereby exposing the device substrate with the mask pattern. Accordingly, if the photomask unfortunately deforms, the mask pattern formed thereon also deforms, which results in the unfortunate deformation of the image of the mask pattern projected to the device substrate as well.
2. Description of the Related Art
Accordingly, to increase the flatness of the photomask, research (See Japanese Patent Application Laid-Open Publication No. 2003-292346, Japanese Patent Application Laid-Open Publication No. 2004-359544 and Japanese Patent Application Laid-Open Publication No. 2005-262432) is being conducted on how to improve the planarity of the substrate through methods of fabrication that eliminate partially protruding portions of the photomask substrate.
Incidentally, in an exposure apparatus that fabricates devices such as FPDs by projecting a mask pattern formed on a photomask to a substrate, thereby exposing the substrate with the mask pattern, it is known to expose the substrate with the mask pattern formed on the photomask while the exposure apparatus holds the photomask substantially horizontally such that the surface whereon the mask pattern is formed faces downward. At that time, as a function of constraints owing to the configuration of the exposure apparatus, the photomask is supported along areas in the vicinity of one set of two opposing sides of the four sides that surround the lower surface of the photomask, namely, the surface whereon the mask pattern is formed. A photomask so supported deforms such that the lower surface of the supported area between the set of sides curves and sags downward owing to its self weight. Here, this deformation is called a first deformation. In addition, simultaneously, the vicinities of the unsupported two sides (i.e., free ends) deform such that they curve and sag downward even farther. Here, this deformation is called a second deformation. There is a risk that such deformation of the photomask might also cause, as explained above, deformation in the mask pattern formed on the lower surface of the photomask.
To accurately form an image of such a deformed mask pattern on the substrate and thereby expose the substrate with the mask pattern, a projection optical system of the exposure apparatus has an autofocus function. Nevertheless, while the depth of focus provided by the autofocus function of the projection optical system covers the first deformation, the effects produced by the second deformation tend to exceed the range of the depth of focus. In addition, as photomasks have increased in size, the second deformation has likewise increased markedly, further departing from the range of the depth of focus of the projection optical system, which is a problem.
Even if satisfactory planarity is achieved as recited in the studies disclosed in Patent Documents 1-3, such an approach cannot solve the problem of deformation that results from the self weight of the substrate.